All fiber lines comprise some type of washing equipment to separate the digestion liquor from the pulp. Later on in the process a washing arrangement is provided to separate bleaching liquors, after the bleaching stages. There are a number of different types of washing equipment which operate according to different principles.
A well-known type of washing arrangement is the drum washer, where the pulp is dewatered on a rotary filter drum after the addition of washing liquid, which displaces the liquor remaining on the pulp web after preceding process stages, for example a digestion stage or a bleaching stage. An underpressure within the drum causes the displaced liquid to pass through a perforated metal sheet located on the rotary drum. A further development of the original drum washer is the pressurized displacement washer, where the filtrate at over-pressure is caused to pass through the metal sheet. The increased pressure difference leads to an improved displacement of the filtrate.
According to a known design of a pressurized displacement washer, the drum is provided with compartments extending in the axial direction of drum and intended to be filled with pulp. The compartments are defined by walls in the form of bars arranged axially along the entire drum shaft, as well as a bottom that consists of the perforated metal sheet. The subdivided compartments of the drum ensure that the pulp cake does not break up and move, but instead maintains the form which is produced upon application of the pulp. The perforated metal sheet, on which the pulp deposits, is located at a distance from the main surface of the drum, so that filtrate channels are formed in the space between the drum and the metal sheet. Along the circumference of the drum there are at least as many filtrate channels as pulp compartments.
In a drum washer, a plurality of different washing stages can be carried out, with separate addition of washing liquid to the different stages, and also re-cycling of filtrate from one stage for use as washing liquid in another stage. In order to achieve maximum washing efficiency the objective is that washing liquid intended for a specific washing stage is not moved to a later washing stage. (Due to a pressure difference between the stages, supplied washing liquid tends to move towards the lower pressure.) In order to be able to separate different washing stages, carried out in one or more washing zones of the drum, and forming stages, carried out in the forming zone of the drum, and discharge stages, carried out in the discharge zone of the drum (enhanced pulp concentration zone constitutes a first part of the discharge zone), the respective zones are sealed by longitudinal (i.e. axial) seals. These longitudinal seals are placed between the rotary drum and the surrounding casing. The filtrates from the respective zones are separated by seals in a peripheral end valve arranged at one or both of the end walls of the drum.
A problem with known drum washers is that the drum during rotation often presents an irregular and even “jerky” operation. It is desirable, both in view of costs and in view of washing effectiveness, that functioning of the drum washer be reliable and that its components be put under as low a load as possible. With an irregular loading, however, the performance of the drum washer runs the risk of being noticeably impaired and in addition the wear on its constituting parts increases.
One object of the present invention is to provide an improved washing apparatus of the type comprising a compartmented drum. In particular, the present invention aims at achieving a more regular running of the washing apparatus when it is in operation and the drum consequently rotates.